Prueba:
DELETE
FROM parim_lang a
WHERE a.lang_hash IN (
SELECT a.lang_hash
FROM parim_lang a
JOIN parim_lang b
ON b.lang_hash = a.lang_hash
AND b.lang_language = SUBSTRING(a.lang_google_translation, LOCATE('-', a.lang_google_translation) + 1)
WHERE a.lang_google_translation REGEXP '^[a-z]+-[a-z]+' ) LIMIT 20
